do back/forward ports (56)

[ThomasWaldmann]

https://github.com/borgbackup/borg/issues?utf8=%E2%9C%93&q=label%3Abackport%2F1.1-maint
https://github.com/borgbackup/borg/issues?utf8=%E2%9C%93&q=label%3Aforwardport%2Fmaster

do 2 simple/small or 1 mid-sized/mid-complex forward/backport from there.

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💰 there is a bounty for this

[ThomasWaldmann]

working on:

• forward port implement BORG_LIBC env variable, fixes #5870 #5876 : while trying to do that, I noticed it does not make sense due to the different way libc is dealt with in master (in comes in via cython's "libc.*" imports - we do not load it via CDLL as before). the libc_name value is not used any more. remove unused libc_name #5883
• for the same reason BORG_LIBC env var is not needed on borg 1.2 any more.
• backport Allow --files-cache=size #5831 : Allow --files-cache=size #5884

[ThomasWaldmann]

Done.